Typically, fuel metering systems for gas turbine engines include an electronic engine control (EEC). The EEC instructs system components (such as valves) to meter fuel to the gas turbine engine as may be required. Some fuel metering systems, known as supervisory systems utilize an EEC to adjust a hydromechanical metering system to provide for the appropriate flight characteristics. Other fuel metering systems, known as hybrid systems, provide a hydromechanical back-up to the EEC to assume the fuel metering function, should the EEC fail or be disabled.
Hybrid fuel metering systems are generally complex including a set of valves controlled by the EEC and a set of valves controlled by the hydromechanical back-up. The EEC provides a signal to a first valve which meters fuel to the engine and provides a signal to a second valve which transfers the metering function to the hydromechanical back-up system should the EEC fail. The hydromechanical back-up system controls a third valve which meters fuel to the engine, a fourth valve which provides a minimum fuel flow to the engine should an engine overspeed condition exist and provides for a mechanical shut-off of the fuel to the engine.
Such hybrid fuel metering systems may be complex, costly, heavy and difficult to engineer. Other hybrid fuel metering systems are sought, and it is to this end that this invention is directed.